These may be classified by the method of cycling from inspiration to expiration. This may be when a preset time has elapsed (time-cycled), a preset pressure reached (pressure-cycled) or a preset volume delivered (volume-cycled). Though the method of cycling is classified according to a single constant, modern ventilators allow a greater degree of control. In volume-cycled mode with pressure limitation, the upper pressure alarm limit is set or the maximum inspiratory pressure controlled. The ventilator delivers a preset tidal volume (VT) unless the lungs are non-compliant or airway resistance is high. This is useful to avoid high peak airway pressures. In volume-cycled mode with a time limit, the inspiratory flow is reduced; the ventilator delivers the preset VT unless impossible at the set respiratory rate. If pressure limitation is not available this is useful to limit peak airway pressures. In time-cycled mode with pressure control, preset pressure is delivered throughout inspiration (unlike pressure-cycled ventilation), cycling being determined by time. VT is dependent on respiratory compliance and airway resistance. Here, too, high peak airway pressures can be avoided.